AXIAL PHASES DISTRIBUTION AND HOMOGENIZATION IN THREE-PHASE FLUIDIZED BEDS

We have investigated the axial distribution of the liquid phase in a three-phase fluidized bed in which the particles were glass beads of two different diameters, 710-850 μm and 2 mm. Two methods were used to reduce the axial heterogeneity and homogenize the bed: the use of packing elements and the addition of a small quantity of a high molecular weight polymer. The use of packing elements was shown to be very efficient in controlling phases distribution in the bed. The packing consisted of 1 mm thick steel horizontal plates punched and stretched to obtain parallelogram-shaped openings. The addition of a high molecular weight polyisobutylene (BASF Oppanol B246, M_u, =6.15 Lo⁶) on the phase distribution was also studied. Low polymer concentrations up to 397 ppm (based on the liquid phase) were investigated. The addition of the polymer reduced bubble coalescence and allowed for a better distribution of the phases.     

The effect of chelating/combustion agent on catalytic activity and magnetic properties of Dy doped Ni–Zn ferrite

The spinel ferrite Ni_0.8Zn_0.2Fe_1.98Dy_0.02O₄ was prepared by sol–gel low temperature autocombustion method using four different chelating/combustion agents: citric acid, tartaric acid, urea and cellulose. Infrared spectroscopy (IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) specific surface area measurement, the catalytic H₂O₂ decomposition and the magnetic behavior were employed to investigate the influence of the combustion agents on structural characteristics, catalytic activity and magnetic properties. Spinel-type phase in the nano-scale domain was accomplished during sol–gel synthesis and was confirmed by XRD and IR. The best catalytic activity is belonging to the sample obtained using urea, which shows the smallest grain size (SEM), the highest specific surface area (BET measurements) and DyFeO₃ phase (XRD), while ferrimagnetic behavior prevails for all the samples independently of fuel agent.     

Preparation and magnetoelectric properties of NiFe2O4–PZT composites obtained in-situ by gel-combustion method

Magnetoelectric composites of xNiFe₂O₄–(1 ? x)Pb(Zr,Ti)O₃ with x = 2, 5, 10, 20, 30% were prepared by citrate–nitrate combustion using PZT-based template powders. In order to ensure a better connectivity of dissimilar phases, we have used chemical methods for preparation in situ composites, followed by adequate sintering procedure. The structural, microstructural and functional properties of di-phase magnetoelectric composites of NiFe₂O₄–PZT are reported. The XRD analysis is demonstrating the synthesis of pure ferrite phase directly on the ferroelectric templates. An excellent mixing was obtained in the composite powders, as proved by a detailed SEM analysis.The magnetic and dielectric behaviors of the ceramic composites vary with the ratio of the two phases. The dielectric behavior is greatly influenced by the magnetic phase. The magnetoelectric (ME) coefficient was measured as a function of applied DC magnetic field. The maximum ME coefficient (dE/dH) varies from 0.0011 mV/(cm Oe) to 0.5 mV/(cm Oe) with increasing of NF addition.     

The stability of boundary-layer flows under conditions of intense interfacial mass transfer: the effect of interfacial coupling

We consider the linear stability of a boundary-layer flow over a permeable surface under conditions of intense interfacial mass transfer. The stability of the flow is governed by an eigenvalue problem of Orr-Sommerfeld type coupled to a second-order differential equation for the concentration disturbance field through a flux boundary condition at the permeable surface. Previous studies on this problem have ignored the effect on the stability of the flow of this coupling. Curves of neutral stability and the critical Reynolds number for the flow are obtained. These show that the fully coupled system produce critical Reynolds numbers and wave-numbers that, in some cases, differ significantly from those obtained when the disturbance coupling is ignored.     

Influence of bubbles characteristics on the skin friction and velocity gradient on solid sphere

A detailed study of the effects of individual bubbles at high gas flow‐rate has shown, that the dominant influence on skin friction over a solid sphere is the bubble volume in compared to bubble frequency. Nevertheless the bubble frequency is very important in case of low gas flow‐rate. Referring to bubbles produced by a gas distributor, statistical and spectral analyses were performed to study the influence of bubbling on exposure time and magnitude of fluctuations. Referring to a calibrated bubble train, the existence of critical frequency is demonstrated. A bubble with larger volume and a mobile, oscillatory surface generates larger velocity gradient. In the case of gas distribution, histograms of the velocity gradient for a 2 mm glass sphere creating bubble coalescence reveal the maximum exceeds 48 000 s^?1 in the front zone and 2000 s^?1 in the rear zone (θ = 180°). For 5 mm plastic spheres creating bubble break‐up, the maximum of the velocity gradient is only 8100 s^?1 for the front part of the sphere and 2000 s^?1 in the rear zone.     

Self-propagating high-temperature synthesis in mechanically activated mixtures of boron carbide and titanium

Self-propagating high-temperature synthesis in both the modes of both layer-by-layer combustion and dynamic thermal explosion was carried out using preliminary mechanical activation of B₄C–Ti powder mixtures in a planetary ball mill. Preliminary mechanical activation conditions of reaction mixtures that provide a reduction in the reaction initiation temperature to 600 ± 20?C were determined. The reaction products consist of mixtures of TiC and TiB₂ with submicron grain size. The results of x-ray diffraction and electron microscopy studies of activated samples and reaction products are presented.